Polar Wander and J̇ ₂ Induced by Ice-Sheet Loading

Abstract

At the end of the 19th century the theoretically predicted wobbling of the Earth’s rotation axis with respect to the Earth’s surface was observed. Two periodic movements have in fact been discerned from astronomical observations: one with a component of one year, related to seasonal influences, and one with a component of about 14 months, which was readily associated with Euler’s free nutation of a rotating deformable body. From astronomical observations, Lambert (1922) and Wanach (1927) derived that the rotation axis of the Earth, besides these two periodic components, also shows a secular (linear) displacement with respect to the Earth’s surface. Lambert (1922) found from an observation time series covering the period 1900–1917 that on the Northern Hemisphere the axis moves in the direction of 90° W longitude with a speed of 1.72 degrees per million years (deg/Myr). On the basis of a somewhat longer time series (1900–1926), Wanach (1927) found a value of 1.31 deg/Myr in the direction 42° W longitude. These observations have been refined as longer time series became available and after the influence of continental drift had been recognized and was corrected for. One of the most recent values comes from McCarthy and Luzum (1996), who report a polar wander speed of 0.925 ± 0.022 deg/Myr in the direction 75.0° ± 1.1° W longitude. To put these values in perspective: the yearly and 14-month harmonic components have amplitudes of about 10–15 meters; the secular component that results after filtering these two harmonic components out has a rate of about 10 centimeters per year (one degree on the Earth’s surface is about 110 km).